Spark plug seal having a lower coefficient of expansion than the ceramic insulator core



Oct. 17, 1967 H. H. ABDELLA 3,343,091

SPARK PLUG SEAL HAVING A LOWER COEFFICIENT OF EXPANSION THAN THE CERAMICINSULATOR CORE Filed April 12, 1965 I N VEN TOR.

ATTORNEY United States Patent O 3,348,091 SPARK PLUG SEAL HAVING A LOWERCOEFFI- CIENT OF EXPANSION THAN THE CERAMIC INSULATOR CORE Harold H.Abdella, Davison, MiclL, assignor to General Motors Corporation,Detroit, Mich, a corporation of Delaware Filed Apr. 12, 1965, Ser. No.447,135 7 Claims. (Cl. 313-136) ABSTRACT OF THE DISCLOSURE Thisinvention relates to a spark plug metal-glass seal, and a method formaking the same. The metal-glass seal composition has a coefficient ofexpansion lower than the spark plug insulator and contains metal powderhaving a particle size between 10 and 100 mesh. This hermetic conductivemetal-glass seal is formed by heating the metalg-lass seal compositionto a softening temperature and subsequently cooling it without theapplication of pressure to the seal composition while it is in asoftened state.

This invention relates to spark plugs and more particularly to a fusedhermetic metal-glass powder seal for use in the centerbore of a sparkplug insulator and a method for making a spark plug having such a seal.

One of the essential requirements of a spark plug is that it be gastight in order to prevent gas leakage from the combustion chamber of theengine in which it operates. Since gas leakage can occur through thespark plug insulator centerbore, it has always been necessary to providesome adequate sealing means between the center electrode or centerwirestructure and the Walls of the insulator centerbore. A fused glasspowder seal is usually used where a one-piece metal centerwire structureis used. Many spark plugs, however, use a centerwire structure having apinrality of metal parts such as a center electrode and a terminal wire.When a plurality of metal parts is used for the centerwire structure, itis the practice to gasproof the centerbore by means of a conductive sealconsisting of a mixture of glass or ceramic material and a conductivemetal powder such as nickel, copper and tungsten.

These conductive seals are formed typically by tamping the metal-glasspowder mixture into the centerbore above and around the centerelectrode, heating this mixture to soften the glass portion and thenapplying pressure with a terminal wire ot the semi-fluid sealcomposition. Upon cooling, the resulting hermetic metal-glass seal bondsto the walls of the centerbore and to the abutting portions of theterminal wire and the center electrode and forms a low resistanceconductive path. It is necessary to apply pressure while the sealcomposition is in the fluid or semi-fluid state with these metal-glassseals in order to obtain a seal which upon cooling is conductive andairtight When such a seal composition is heated to the requiredtemperature, the semifluid glass flows around the metal particlescompletely encasing the metal particles. Metal particles which arecompletely encased by the glass are not conductive. When the terminalwire is pressed into the heated seal composition, the pressure forcesthe fluid glass mass from between the metal particles outwardly towardthe insulator centerbore walls. As a result, the metal powder particlesare in contact with one another thereby forming a conductive path in theseal. Moreover, the mass of glass near the centerbore walls has beenappreciably increased thereby wetting the walls sufficiently so that ahermetic seal is formed upon cooling. Therefore, a pressure applied tothe fluid metal-glass seal mass effects both a conductive seal and ahermetic seal. Thus, prior to the applicants invention, the. applicationof pressure to a heated metalglass seal composition was required toyield a ueal which was conductive and air-tight.

The design of spark plugs requiring a. hermetic conductive metal-glassseal has been restricted because of the necessity for applying apressure to such a seal. In view of this, it can be seen that a needexists for a metalglass seal composition which would form a hermeticcondutive seal without the application of pressure.

It is a basic object of this invention to provide a spark plugcenterbore metal-glass seal composition which will provide a conductiveseal without the application of pressure to the seal composition whileit is in the fluid or semifluid state.

It is another basic object of this invention to provide a spark plugcenterbore metal-glass seal composition which will provide a conductiveseal which is air-tight without the application of pressure to thecomposition while it is in a fluid or semi-fluid state.

It is yet another object of this invention to provide a method of makinga spark plug having a hermetic conductive seal Without the applicationof pressure to the seal composition while it is in the fluid orsemi-fluid state.

These and other objects are accomplished by a method of manufacturing aspark plug comprising the steps of inserting a centerwire into thecenterbore of a spark plug insulator body, and loading and packing alayer of metalglass powder mixture which covers the centerbore head Thismetal-glass powder mixture has a coefiicient of thermal expansion lowerthan the insulator and contain a metal taken from the group consistingof Kovar metal alloy, tungsten and molybdenum. The three essentialrequirements for a metal-glass seal to be operative in this inventionare that the glass powder must have a coeflicient of thermal expansionlower than the insulator material, that the metal powder must have acoefficient of thermal expansion lower than the insulator material andthat the metal powder contain 35 weight percent or more metal particleshaving a particle size smaller than 10 mesh and larger than mesh. Alayer of metal powder is then loaded and packed on top of themetal-glass powder layer. Subsequently, the spark plug assembly isheated until the metal-glass powder becomes at least semi-fluid. Uponcooling this spark plug assembly, the fused metal-glass powder hasformed a conductive hermetic seal in the spark plug insulatorcenterbore.

Other objects and advantages of this invention will be apparent from thefollowing detailed description, reference being made to the accompanyingdrawing'wherein'a preferred embodiment of this invention is shown.

The drawing is -a longitudinal cross-sectional view of a spark plugembodying the invention. This drawing represents a visual indicatingspark plug which is the subject of the copending patent application S.N.447,318 filed simultaneously with this application and assigned to theassignee of this invention.

Referring now to the drawing, the spark plug 10 comprises a conventionalouter metal shell 12 having a ground electrode 14 welded to the lowerend thereof. Positioned within themetal shell 12 and secured in theconventional manner is an insulator '16. The insulator 16 is formed witha centerbore having a lower portion 18 of relatively small diameter, alower center portion 20 of larger diameter, an upper center portion 22of still larger diameter, and an upper portion 23 of yet still largerdiameter. Near the top of the upper center portion 22 the insulator haszones of relatively thin cross sections as shown by annular rings 24.

Positioned in the lower portion 18 of the insulator centerbore is thecenter electrode 28, the serrated lower end thereof projecting beyondthe lower tip of the insulator 16. The center electrode head 30 rests oninsulator centerbore ledge 32 connecting the lower center portion to thelower portion 18 of the insulator centerbore.

A metal-glass seal 34 forms a hermetic conductive seal between thecenter electrode head 30 and the insulator centerbore wall of the lowercenter portion 20. The composition of this metal-glass seal 34 must besuch as to form a hermetic conductive seal without being pressed by aterminal screw or other means as is the usual practice; therefore, thiscomposition, upon heating alone, must form a hermetic conducitve seal. Acritical component of this'metal-glass seal 34 is the metal. Thecoefiicient of expansion for the metal must be lower than the insulatorand similar or lower than the glass thereby permitting the glass and themetal of this metal-glass mixture to expand upon heating and contractupon cooling at a lower rate than the insulator. The metal and glassshould have a lower coefficient of expansion than the insulator materialso that the insulator exerts a small compressive pressure on themetal-glass mixture at all times. Metals having a coefficient ofexpansion lower than the insulator which may be used are molybdenum,tungsten and Kovar metal alloy. Kovar metal alloy is used in thepreferred embodimerit. Kovar is a low expansion, iron based alloy with28 to 30% of nickel and 15 to 18% cobalt and fractional percentages ofmanganese. The amount of metal powder in the metal-glass mixture rangesfl'om 55 to 70%, the lower figure indicating the minimum amount of metalrequired to adequately conduct the electrical current and the higherfigure making provision for the minimum amount of glass needed forfluidity and sealing purposes.

It is important that the metal and the glass have a similar coeflicientof expansion in order to obtain a seal which is air-tight. When themetal and glass have a similar coefficient of expansion the homogenousmetalglass mixture expands at the same rate upon heating and in doing sothe relative position of the. metal powder in relationto the glassremains the same. Moreover, as the seal composition contracts uponcooling at the same rate, the components contract at the same ratethereby maintaining the same relative position of the metal powder inrelation to the glass. In other'words, the relative position of themetal and glass remains the same during the entire heating and coolingcycle, the relative position of the metal to glass being one of physicalcontact during the entire heating and cooling cycle. This similarity inthe coeflicients of expansion between the glass and the metal permitsthe glass metal mixture to expand as a unit or mass thereby avoiding theformation of any internal stresses or voids within the mass.

The following table lists the pertinent linear coefiicient of expansiondata. i Linear coeflicient of As shown in the table, the coefficient ofexponsion of the borosilicate glass and the Kovar metal alloy used inthe preferred embodiment is 6.5 and 6.0 respectively. While thecoefiicient of expansion value of Kovar is preferred, the coefficient ofexpansion values of tungsten and molybdenum are sufiiciently close tothat of the borosilicate glass to be operative in this invention.

When a copper-glass mixture, copper having a coeflicient of thermalexpansion about three times as great as glass, is heated and cooled theresulting mass has stresses and voids therein. As the copper-glassmixture is heated, the copper expands at a fa'ster'rate and tends topull or draw away from the glass mass which in turn would leave voidspaces near the copper particles. When this copperglass mixture iscooled, the copper will contract at a faster rate and those areas ofcontact between the glass and the copper which still existed at the hightemperature would be subjected to the copper metal shrinking or pullingaway from the glass-copper contact areas thereby creating more voidspaces or regions of stress. As ifldlcated by the above description, theresulting mass of copper-glass mixture which had been heated and cooledwould result in a mass containing anumber of void spaces and regionsunder stress. Such a metal-glass mass having these voidspaces would havea tendency to leak. In addition, the regions under stress would besubject to fracturing and leaking while in use The use of a metal havinga coefficient of expansion similar to glass eliminates the voids andstresses discussed above,

The metal powder must contain at least 35 weight er. cent particleshaving a size smaller than 10 mesh and larger than mesh as measured bythe Tyler standard screen scale in order to obtain electricalconductivity in the fused metal-glass and in obtaining a hermetic seal.The particles referred to above would pass through a 10 mesh screen butwould not pass through a 100 mesh screen. As discussed earlier, themetal powder typically used in conductive metal-glass seals, for example250 I mesh size, is completely coated on all sides by the glass when theglass becomes heated to a semi-fluid state. However, when the metalparticles are larger, for example minus 35' to plns l00mesh, the largersize of the metal particles has a tendency to inhibit the glass fromcompletely coating it. In other words, the minus 35 to plus 100 meshsize particles are so bulky that when the glass flows it does not coverthe metal particles completely. As a result, some of the non-glasscovered metal particl v portions come in contact with other non-glasscovered metal particle portions. This contact by the non-glass cov redmetal particles makes the seal conductive. It has been observed that notall of the metal particles have to be larger than 100 mesh size to makethe seal conductive. In the preferred embodiment, Kovar metal powdercon-. taining 36 weight percent of particles having a minus 35 to plus100 mesh screen size has been found to work very satisfactorily.However, Kovar metal powder which did not have any metal particleslarger than 100 mesh size did not work satisfactorily in this invention.The concentration range of metal particles having a size smaller than 10mesh and larger than 100 mesh required for this invention is from 35weight percent to 100 percent of the metal powder.

The presence of metal particles which are larger than 100 mesh size alsoenables the glass to sufl'iciently wet the insulator walls therebyforming a hermetic bond between the insulator walls and the metal-glassmass. It is believed that the size of these large metal particlespermits only a certain number of these large metal particlesto be inphysical contact with the insulator walls. As a result, there is asufficient amount of glass located between these large metal particles.to adequately wet the insulator walls to insure an air-tight bondbetween the metal-glass mass and the insulator Walls. 0 7

As has been mentioned earlier, the glass used in this metal-glass sealcomposition should have a coefficient of expansion lower than theinsulator. The glass must have a coeflicient of expansion lower than theinsulator and similar to or somewhat higher than the metal in order toform an air-tight bond with the insulator wall and to prevent theformation of voids and stresses within the metal-glass mass. Aborosilicate glass powder, Corning 7052 glass, having a composition of65% SiO- 23% 0 5% A1 0 and 7% o works well in this metalglass seal. Theamount of glass powder in the'metalglass ranges from 30 to 45%.The meshsize of the glass powder does not appreciably affect the performance ofthe seal, a mesh a size of 200 for the glass powder was found to besatisfactory. I

Other components in a metal-glass seal composition are a ilux and abinder. The amount of flux in themetal-glass mixture ranges from 1 to4%. Boric anhydride is used in the preferred embodiment although anyborate glass may be used. A binder which prevents oxidation of the metalis burned off during the heating of the metal-glass mixture. The amountof binder in the mixture ranges from 1 to 3%. Hydrogenated cottonseedoil was used in the preferred embodiment although other binders such ascarlgoxyl methyl cellulose, 1201 wax, and dextrin may be use Ametal-glass powder mixture which worked well is as follows:

Hydrogenated cottonseed oil 3 On top of the metal-glass seal 34 is alayer of metal powder 36 which serves the lower electrode in theinsulator body chamber. The lower electrode 36 may be any conductivemetal such as copper, nickel, iron, platinum, silver, tungsten, andKovar with Kovar powder being used in the preferred embodiment. Abovethe lower electrode 36 is a sealed insulator body chamber 38 whichextends up to the electrode and exhaust stem assembly 40. The insulatorbody chamber 38 contains an inert gas. The el trode and exhaust stemassembly 40 consists of a Kovar metal tube whose lower portion extendsoutwardly in a conical fashion similar to a funnel so that the lowerflanged end of the tube has a diameter similar to the diameter of theinsulator centerbore at the position of the ledge 41. The flanged end ofthe electrode and exhaust stem assembly 40 is positioned on top of theinsulator centerbore ledge 41, ledge 41 connecting the upper centerportion 22 with the upper portion 23 of the insulator centerbore. Theupper end of the electrode and exhaust stem assembly 40 is welded shutthereby making that end of the assembly 40 air tight. Glass seal 42forms a hermetic seal between the upper surface of the lower portion ofthe electrode and exhaust stem assembly 40 and the insulator centerborewall of the upper portion 23.

The terminal 44 encloses and covers the upper portion of the electrodeand exhaust stem assembly 40 and fits over the top end of the insulatorbody 16. The lower end of the terminal 44 is crimped over the insulatorbody under rib 43 thereby locking the two elements tightly together. Theterminal 44 may be in electrical contact with the electrode and exhauststem assembly 40, in which case, the current is conducted through theelectrical contact. However, if the terminal 44 is not in electricalcontact with the electrode and exhaust stem assembly 40 the currentjumpsthe gap between the terminal 44 and the electrode and exhaust stemassembly 40.

During the sparking operation of the visual indicating spark plugdescribed in the drawing, the current passes from the spark plugterminal 44 to the electrode and th exhaust stem assembly 40. As thecurrent traverses the chamber 38 separating the electrode and exhauststem assembly 40 from the metal electrode 36, the inert gas isenergized, thereby resulting in a glowing insulator body which projectslight through the translucent insulator wall 24 to indicate to anobserver the presence of a high voltage in the spark plug. The currentthen passes from the metal electrode 36 through the metal-glass seal 34,through the centerwire electrode 28, across the gap to the sideelectrode 14, and then to the shell 12 where it is grounded to theengine block (not shown).

This invention will now be described in terms of a method for making aspark plug having a hermetic conductive metal-glass seal in theinsulator centerbore without the application of pressure to the sealcomposition while it is in a fluid or semi-fluid state. As shown in thedrawing, the centerwire electrode 28 is inserted into the centerbore ofthe insulator body 16. A centerwire electrode head 30 rests upon theinsulator ledge 32. A metal-glass seal powder load 34 ranging from 0.25to 0.35 gram is placed on top of the centerwire head 30 in thecenterbore por tion 20 in the conventional manner and tamped. Thecomposition of this metal-glass powder 34, as discussed in detailearlier, must have a coefficient of expansion lower than the insulatorbody and must contain'metal particles having a particle size smallerthan 10 mesh and larger than mesh. A layer of 0.10 to 0.20 gram of Kovarmetal alloy powder 36 is placed on top of the metal-glass powder layer34 and tamped. Other metal powders, such as iron, nickel, and so forth,may be used for the metal powder layer 36. Subsequently, the insulatoris heated in air at approximately 1740 F. for 10 minutes in order tosoften the metal-glass powder layer 34. Upon cooling, the fusedmetal-glass powder layer 34 has formed a hermetic conductive metal-glassseal which conducts electricity from the center electrode 28 to themetal powder layer 36, all three elements being a part of the centerwirestructure. Other steps which are well known in the art may be followedbefore and after the heating step in order to complete the spark plug.

The term fused as used herein is intended to encompass softening up toand including liquid-faction.

While the invention has been described in terms of a specificembodiment, it is to be understood that the scope of the invention isnot limited thereby except as defined in the following claims.

I claim:

1. A spark plug comprising an insulator having a centerboretherethrough, a fused hermetic metal-glass seal of high electricalconductivity in said centerbore, said seal being positionedperpendicularly to the longitudinal axis of said centerbore and bondedto the insulator Walls to prevent the flow of gas through thecenterbore, said seal having a coeflicient of expansion lower than saidinsulator which is suflicient for said insulator to exert a compressiveforce on said seal, said seal containing metal powder having metalparticles having a size smaller than 10 mesh and larger than 100 mesh,said metal particles ranging from 35 weight percent to 100 weightpercent of the metal powder.

2. A spark plug comprising an insulator head and a centerboretherethrough, a fused hermetic metal-glass seal of high electricalconductivity in said centerbore, said seal being positionedperpendicularly to the longitudinal axis of said centerbore and bondedto the insulator walls to prevent the flow of gas through thecenterbore, said seal having a coeflicient of expansion lower than saidinsulator which is suflicient for said insulator to exert a compressiveforce on said seal, said seal comprising a metal powder having acoeflicient of expansion lower than aid insulator, said seal containingmetal powder having metal particles having a size smaller than 10 meshand larger than 100 mesh, said metal particles ranging from 35 weightpercent to 100 weight percent of the metal powder.

3. A spark plug comprising an insulator having a centerboretherethrough, a fused hermetic metal-glass seal of high electricalconductivity in said centerbore, said seal being positionedperpendicularly to the longitudinal axis of said centerbore and bondedto the insulator walls to prevent the flow of gas through thecenterbore, said seal having a coeflicient of expansion lower than saidinsulator which is sufiicient for said insulator to exert a compressiveforce on said seal, said seal comprising a metal powder having acoefiicient of expansion lower than said insulator and a glass having acoeflicient of expansion lower than said insulator, said seal containingmetal powder having metal particles having a size smaller than 10 meshand larger than 100 mesh, said metal particles ranging from 35 weightpercent to 100 weight percent of the metal powder.

4. A spark plug as described in claim 1 wherein said metal is taken fromthe group consisting of Kovar, tungsten and molybdenum.

5. A spark plug comprising an insulator having a centerboretherethrough, a fused hermetic metal-glass seal of high electricalconductivity in said centerbore, said seal being positionedperpendicularly to the longitudinal axis of said centerbore and bondedto the insulator Walls to prevent the flow ofgas through the centerbore,said seal having a coelficient of expansion lower than said insulatorwhich is sufficient for said insulator to exert a compressive force onsaid seal, said seal comprisirrg 55 to 70% of a metal taken from thegroup Consisting f Kovar, tungsten and molybdenum and the balanceSubstantially glass, said seal containing metal powder having metalparticles having a size smaller than mesh and larger than 100 mesh, saidmetal particles ranging from 35 weight percent to 100 weight percent ofthemetal powder." i Y i 6. A spark plug comprising an insulator having acenterbore therethrough, a center electrode positioned in the lower endof said insulator centerbore, a fused hermetic metal-glass sealpositioned in said centerbore perpendicularly tothe longitudinal axis ofsaid centerbore above and'around the upper portion of said centerelectrode and bonded to said insulator and said center ele trode, saidseal having a coeflicient of expansion lower than said insulator Whichis sufiicient for said insulator to exert a compressive force on saidseal, said seal comprising 55 to 70% of a metal talcen from the groiipconsisting of Kovar, tungsten and molybdenum and the balancesubstantially glass, said seal containing metal powder having metalparticles having a size smaller than 10 mesh andlarger than, 100 mesh,said metal particles ranging from 35 weight percent to 100 Weightpercent of the metal powder, 30 to 45% glass having a coefficient ofexpansion lower than said insulator, 1 to 4% flux and 1 to 3% binder. i

7. A spark plug comprising an insulator having a centerboretherethrough, a center electrode positioned in prising to of a metalpowder taken from the group consisting of Kovar, tungsten, molybdenumand the balance substantially glass, said seal containing metal powderhaving metal particles having a size smaller than 10 mesh and largerthan mesh, said metal particles ranging from 35 weight percent to 100Weight percent of the metal powder, 30 to 45 glass having a coeflicientof expansion lower than said insulator, 1 to 4% flux and 1 to 3% binder,a metal powder layer positioned in said center, bore above and inelectrical contact with said seal, said seal conducting electricitybetween said center electrode and said metal powder layer.

References Cited UNITED STATES PATENTS 2,248,415 7/ 1941 Schwartzwalderet a1. 7 313,136 X' 2,988,662 6/1961 Smith 313-4 24 X JAMES W. LAWRENCE,Primary Examiner. DAVID I GALVIN, Examiner.

o. R. CAMPBELL, Assistant Examiner.

1. A SPARK PLUG COMPRISING AN INSULATOR HAVING A CENTERBORETHERETHROUGH, A FUSED HERMETIC METAL-GLASS SEAL OF HIGH ELECTRICALCONDUCTIVITY IN SAID CENTERBORE, SAID SEAL BEING POSITIONEDPERPENDICULARLY TO THE LONGITUDINAL AXIS OF SAID CENTERBORE AND BONDEDTO THE INSULATOR WALLS TO PREVENT THE FLOW OF GAS THROUGH THECENTERBORE, SAID SEAL HAVING A COEFFICIENT OF EXPANSION LOWER THAN SAIDINSULATOR WHICH IS SUFFICIENT FOR SAID INSULATOR TO EXERT A COMPRESSIVEFORCE ON SAID SEAL, SAID SEAL CONTAINING METAL POWDER HAVING METALPARTICLES HAVING A SIZE SMALLER THAN 10 MESH